Reactive silicon group-containing polymers undergo crosslinking curing in the presence of moisture. Among such polymers is, for example, a curable composition which contains a polymer having a main chain skeleton formed of a polyoxypropylene, has characteristics such that it is liquid at room temperature and turns into a rubbery elastic substance by curing, and is accordingly widely used as sealants for use in building construction.
Various physical properties are demanded when curable compositions are used in building construction, and among such properties is a property that the staining around the sealing joints can be suppressed.
As one of the causes for the staining around joints, the bleeding of the plasticizers contained in the sealants may be cited. However, exclusion of the plasticizers leads to a problem that the physical properties of the cured product tend to be high in modulus and low in elongation, resulting in the quality degradation and other failures of the sealants. Also, in the case where the average number of the reactive silicon groups contained in one molecule of the polyoxyalkylene polymer is decreased for the purpose of lowering the modulus of the plasticizer-free cured product, there occurs a problem that the proportion of the unreacted polyoxyalkylene polymers having absolutely no reactive silicon groups is increased, and such polyoxyalkylene polymers cause staining around joints and other failures, similarly to the plasticizers.
For such problems, Patent Document 1 (Japanese Patent Laid-Open No. 5-65403) discloses a curable composition which includes an oxypropylene polymer, containing at least one reactive silicon group and having Mw/Mn=1.6 or less and a number average molecular weight of 6,000 or more, a filler and a curing catalyst, but does not include a plasticizer, wherein the dust adhesion of the curable composition is improved while the workability thereof is being maintained. Examples 1 and 2 of Japanese Patent Laid-Open No. 5-65403 also present curable compositions each of which includes an oxypropylene polymer, having a number average molecular weight of 10,000 and Mw/Mn=1.2 and containing approximately 2.4 reactive silicon groups in one molecule of the oxypropylene polymer, a filler and a curing catalyst, but does not include a plasticizer.